Understanding Storm Track Position and Intensity Across a Range of Timescales

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Warm days preceding and cold days following the passage of frontal weather systems are a familiar feature of winter weather in the continental US and other middle latitude regions. This alternation is important not only for local weather but for the global transport of heat and moisture, as warm moist air crosses the middle latitudes from the tropics to the poles while colder and drier transits in the opposite direction. This north-south exchange of heat and moisture plays an essential role in earth's climate, keeping the poles from getting too cold while limiting the temperature excess of the tropics. Such climatic effects of weather systems are commonly studied by looking at the midlatitude stormtracks, defined as zones of maximum weather system activity. In the Northern Hemisphere separate stormtracks are centered over the northern North Pacific and North Atlantic, while in the Southern Hemisphere a continuous stormtrack extends around the "roaring 40s" over the uninterrupted Southern Ocean.This project seeks to understand how the mean latitude of the stormtracks, and the and strength of the energy transport by the weather systems moving along them, change in response to external factors including the seasonal cycle of solar heating, greenhouse gas concentrations, stratospheric aerosols, ice age cycles, and the distribution of continents and oceans. The research is conducted using a theoretical framework developed by the Principal Investigator (PI) in which stormtracks are characterized in terms of the flux of moist static energy (MSE) across them, where MSE is a measure of energy which includes the thermal energy associated with air temperature, the latent energy of water vapor released as heat during condensation, and the potential energy of air parcels lifted against the force of gravity. The MSE flux in stormtracks works in concert with MSE flux in circulations around large stationary pressure centers such as the Aleutian Low and the Bermuda High (referred to as stationary waves), and the circumglobal meridional overturning circulations (the Hadley and Ferrel cells), to transport energy from the tropics to the poles and balance the atmospheric energy budget. Much of the previous research on stormtracks has been conducted through analysis of the momentum budget and related quantities (potential vorticity and Eliassen-Palm flux, for instance), thus the MSE analysis of the present work offers a novel and complementary perspective.Preliminary work by the PI and colleagues shows compensation between MSE flux in stormtracks and stationary waves as a prominent feature of the seasonal cycle and the midlatitude response to El Nino events, while compensation between MSE flux in stormtracks and meridional overturning circulations appears in simulations of the atmopsheric response to greenhouse gases and the radiative effects of aerosols. The present work extends these investigations and seeks to determine the fundamental dynamics through which these compensations, in which changes in stormtrack MSE flux are accompanied by opposing changes in other forms of MSE flux, come about. The work is conducted through a combination of observational analysis and simulations with numerical models at varying levels of complexity.The work has societal relevance as well as scientific interest due to the importance of stormtrack behavior for human activities in the midlatitudes, including the continental US. In addition, one goal of the research is the identification of "emergent constraints" which can be used to assess the credibility of climate change simulations. Emergent constraints are relationships that emerge between model-to-model variations in simulated present-day climate (the predictor) and model-to-model differences in simulated future climate change (the predictand). Such constraints are used to assess the credibility of climate change projections, as models which incorrectly represent the predictor, which can be compared to real-world observations, are likely to produce comparable misrepresentations in their simulations of future climate change. Such tests of climate change projections are valuable given the use of climate models to inform climate change adaptation efforts. The project also supports a graduate student and the participation of an undergraduate, thereby providing for the education and training of the future workforce in this research area.